Conventionally, portable miniature label printers have a single print head, such as a thermal printhead, for enabling printing on a single roll of paper or label stock. Examples of portable single printhead printers are described in U.S. Pat. Nos. 5,267,800, 5,806,993, and 5,594,838. One drawback of such printers is that they are limited to printing on only one type of paper at a time from the roll in the printer. Thus, a user of the printer must switch between rolls of different types of paper, which requires time and dexterity to open and reload the paper in the printer. Further, the printer may need to be reprogrammed to provide printing on different types of labels. To avoid switching between rolls, multiple label printers can alternatively be carried by a user. However, this is expensive as it requires purchasing multiple portable printers capable of printing on different rolls. Thus, it would be desirable to provide a label printer having two printheads which are capable of printing on two different types of paper from two different rolls.
Two printheads have been used in Point of Sale (POS) stations in the retail industry, such as described for example in U.S. Pat. No. 5,782,567. These POS stations often have printing devices limited to enabling printing of customer receipts from a roll of paper, and a record of each sale on another roll of paper which is stored on a take-up reel in the printer. Similarly, U.S. Pat. No. 4,747,707 describes a label printer having a first printing device for printing a label, and a second printing device for printing on record paper stored on a take-up shaft in the printer. The second printing device in each of these patents is limited to recording the same information as the first printing device, and thus is not operated independently from the first printing device. Furthermore, as only one of the printing devices can actually print a label for a user, this printer has the same drawback as the single printhead label printer.
Further, the printhead in a portable label printer is fixably mounted and thus cannot be easily changed or replaced by the typical user if either the printhead is damaged or a different width printhead is needed. Accordingly, it is further desirable to provide a portable printer in which the entire printing mechanism, including the printhead, is in a module which can easily be replaced with another module having another printing mechanism.